The present invention generally relates to a transflective type IPS type color liquid crystal display device. More specifically, the present invention is directed to a phase difference layer of a reflection display portion of the color liquid crystal display device.
Portable type information appliances which are typically known as cellular phones will be firmly progressed with having multiple functions and high performance also in near future, while storage capacities of storage devices thereof are increased and communication speeds thereof are increased. Also, with respect to display devices functioning as interfaces for these portable type information appliances, these display devices are necessarily required to be drivable in response to larger capacities of image information. To this end, these display devices are required to be capable of realizing high image qualities as well as high precision displays. More concretely speaking, as to the high image qualities, high contrast ratios, high color reproducible characteristics, wide view angles, and outdoor visibility may be listed up.
While the portable type information appliances may be utilized under various sorts of environments, these information appliances are operable under the direct rays of the sun in midsummer as a limit of light environments, whereas these information appliances are operable in darkrooms as a limit of dark environments. In order to achieve relatively better displays under all of these various environments, there are many possibilities that transflective type liquid crystal devices are employed.
IPS (In-Plane Switching) type liquid crystal display devices are capable of displaying thereon images at wide view angles and in high contrast ratios. Such transflective IPS type liquid crystal display devices have been proposed by modifying the above-described IPS type liquid crystal display devices.
On the other hand, very recently, while material techniques of coating type phase plates have been progressed, if these coating type phase plates are selectively formed on reflection display portions contained in pixels of transflective type liquid crystal display devices, then transmission display portions can be made to have the same structures as those of transmission type IPS systems. As a result, in the transflective type liquid crystal display devices, the reflection display portions are capable of displaying thereon display contents at wide view angles and in high contrast ratios, which are similar to those of the transmission type IPS systems. The resulting liquid crystal display device will be referred to as a “phase plate built-in type transflective IPS liquid crystal display device.”
Generally speaking, in a reflection type liquid crystal display device, a polarization condition of light which reaches a reflection layer is converted to a circularly polarized light condition, so that a dark display may be realized. The basic idea of such a dark display may be similarly applied to a reflection display portion of a transflective type liquid crystal display device, and furthermore, may be similarly applied also to a reflection display portion of a phase plate built-in type transflective IPS liquid display device.
Assuming now that one pair of substrates which sandwiches a liquid crystal layer are constituted by a first substrate and a second substrate, there is such a selection that a phase difference layer is formed on any one of the first and second substrates. Assuming now that such a substrate which is located near a light source for irradiating visible light onto the liquid crystal layer is defined as the second substrate and another substrate is defined as the first substrate, the second substrate has various sorts of the wiring lines, electrodes, and active elements, so that a layer structure of this second substrate is considerably complex, as compared with a layer structure of the first substrate having a color filter. As a consequence, it is readily possible to form the phase difference layer on the side of the first substrate having the simpler layer structure. As a result, the liquid crystal layer is arranged in the vicinity of the reflection layer, as compared with the phase difference layer.
The light which reaches the reflection layer is converted into circularly polarized light by employing this phase difference layer and the liquid crystal layer so as to realize a dark display. In this case, in accordance with a technical idea of a broadband circular polarizing plate, if the liquid crystal layer located near the reflection layer is constructed as a quarter-wave plate and the phase difference layer is constructed as a half-wave plate, then either circularly polarized light or such a polarization condition close to this circularly polarized light may be realized with a wide range of a visible wavelength range.
If a reduction of a reflectance of a dark display has a top priority, then a design for a reflection display portion of a phase plate built-in type transflective IPS liquid crystal display device may be exclusively determined based upon this factor.
If the liquid crystal layer is made of the quarter-wave plate, then a retardation value thereof must be selected to be approximately 140 nm. As a result, even when such a liquid crystal material having the smallest birefringent value is employed, a thickness of the liquid crystal layer must be made smaller than, or equal to 2 μm. In this case, an influence caused by orientation control force of an orientation film is large which is given to the liquid crystal layer, and thus, an orientation change of the liquid crystal layer can hardly occur when a voltage is applied thereto. The orientation change of the liquid crystal layer when an electric field is applied is small, and a reflectance of light display is lowered.
In order to improve the reflectance of the light display, the thickness of the liquid crystal layer of the reflection display portion must be increased. However, at this time, the retardation value of the liquid crystal layer of the reflection display portion is considerably larger than the quarter wavelength, so that an ideal broadband circularly polarizing plate cannot be formed. As a secondary solution idea capable of lowering the reflectance of the dark display, it is conceivable that the retardation value of the phase difference layer is increased in conjunction with the retardation value of the liquid crystal layer. If thickness of the liquid crystal layer of the reflection display portion is slightly increased, then a relatively lower reflectance of the dark display may be achieved. However, the reflectance of the dark display is increased in connection with such a condition that the retardation value is increased from the quarter wavelength.
If light having such a wavelength near a wavelength of 550 nm where a visual sensitivity becomes maximum is designed to become circularly polarized light, then a reflectance of luminance brightness can be reduced. As a result, in the most case, a retardation value is designed, while the wavelength near the wavelength of 550 nm is defined as a reference.
In the case of the phase plate built-in type transflective IPS liquid crystal display device, in particular, an increase in reflectance of a short wavelength range becomes conspicuous in connection with such a condition that the thickness of the liquid crystal layer of the reflection portion is increased. Although the reflectance of the light display may be increased, the reflectance of the dark display is increased as a secondary effect thereof, and furthermore, the liquid crystal display is colored in blue. For instance, JP-A-2006-39369 (will be referred to as “patent publication 1” hereinafter) has described such a transflective type liquid crystal display device that retardation values of built-in phase plates are changed with respect to each of respective color filters. However, the transflective type liquid crystal display device of the patent publication 1 is not made as an IPS type liquid crystal display device, so that transmission displays having high contrast ratios cannot be achieved in wide visual angles.